Will someone at Home Depot or Lowe's be able to cut this for me? If not, what tool should I buy and use?

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Thanks for all the help! I ended up buying a Dremel. I went through 6 cut-off wheels in order to finish all my cuts. I was customizing an IKEA drawer to fit under my smaller IKEA table. Here is one of my noob cuts:

Is this aluminum channel, or is it steel? Will a magnet stick to it? This is important to know, since that would influence my choice of tools. A magnet will stick to steel. (I'm pretty confident this is not stainless steel from the looks of it, but I might be wrong. If so, then a magnet will not stick to some grades of stainless. And stainless offers some difficulties of its own for a novice to cut.) This looks like a steel doorframe channel.

For example, on steel, I'd go with an angle grinder as the fastest way to make the cuts, and a way to make a fairly straight cut. But aluminum would gum up the cutoff wheel.

For aluminum, I'd choose an oscillating multitool, like that from Porter Cable, Dremel, Fein, Bosch, etc. These tools take a toothed blade that oscillates rapidly, and will cut through aluminum easily. Again, you will get a fairly straight cut with the proper choice of cutting blade.

Lacking either of these tools, or being unwilling to buy one, I'd go with a hacksaw, which can cut either metal. Of course, a hacksaw will yield a somewhat jagged cut unless you have some practice here.

One problem with the cut will be in minimizing any vibrations of the metal as it is being cut. A jigsaw, or oscillating multitool, or hacksaw will all cause the material to bounce around. A good vise will help solve this. In fact, my preference would be to clamp a piece of sacrificial wood inside the channel, then put it all in a vise. That will yield the best and most vibration free (therefore cleanest) cut possible.

In any event, I'd expect to need to clean up the edges, rounding them so there are no sharp edges. A file will do for a start, then I'd switch to some emory paper. (Ok, sandpaper in a pinch.)

If the material is steel, make sure you paint the surface afterwards with a rust inhibitor, as the cut edges will rust otherwise.

I personally would use an angle grinder because it's a lot easier to keep a straight line. That, and you can use the same tool to clean the burrs off after. However, they're more dangerous than a jigsaw. You could always start out with a dremel and a few cutoff wheels.

I mentioned burrs above. Since it looks like you've never cut metal before, be aware that after you cut the surface will be very rough and sharp. You'll want to file or grind down the surface to make it safe.

The cleanest and fastest way would be to use a power jigsaw with an appropriate blade ("for metal", small teeth). You'll still need to drill two holes in the corners to be able to turn the blade in between cuts. You'll then need a file for minor final treatment of the cuts.

The cheapo way would be to use a hacksaw for the lengthwise cuts and then a series of drilled holes along the crosscut, then use cutting pliers to cut the leftover metal between the holes and a file to remove the major burrs. You can use the drill-cut-file technique for the lengthwise cuts too. If you follow this way leave some extra space between where you drill and where the cut needs to be - some metal will be wrecked into chaos and you'll need to file it away.

Home Depot usually won't cut material that is not purchased new from their store.

if vise then hacksaw

If you can clamp the area you need to cut in a vise, I would use a hack saw. Use some corrugated cardboard in the jaws to protect the finish. You may want to watch some YouTube videos on how to properly use a hacksaw (yes there are good and bad techniques).

if no vise then jigsaw

If you can't use a vise, I would use a jig saw.

Blade Selection is Important

For both tools, selecting a blade with the right tooth profile and pitch for the material is important. The symbols on the blade packages you'll find at home depot or lowes will help guide you to the right blade. You should probably err on the side of more teeth per inch because your material looks fairly thin.

Not the cheapest, but by far the easiest: Metal cutting circular saw. They run at a little lower speed than a regular skilsaw, and they use a carbide-tipped blade with a much less aggressive rake angle.

Mine came from Harbor Freight, I paid about $90 for it. It'll cut 1/4" plate like butter. For straight cuts it's much faster and cleaner than plasma. Seriously. It produces hardly any sparks, just a whole bunch of blued little metal chips. It seemed too good to be true until I used it for myself, but it really works. There are few videos on Youtube that show just how easily this thing cuts steel.